Method for repairing titanium alloy components

ABSTRACT

A method for repairing a titanium alloy surface of a turbine component includes the step of cold gas-dynamic spraying a powder material comprising at least one titanium alloy directly on the titanium alloy surface. The method may further include the steps of hot isostatic pressing the cold gas-dynamic sprayed turbine component, and performing a separate heat treating step after the hot isostatic pressing. Thus, the cold gas-dynamic spray process and post-spray processing can be employed to effectively repair degraded areas on compressor turbine components.

TECHNICAL FIELD

The present invention relates to repair and overhaul of turbine enginecomponents. More particularly, the present invention relates to methodsfor repairing turbine engine components made from titanium alloys.

BACKGROUND

Turbine engines are used as the primary power source for many types ofaircrafts. The engines are also auxiliary power sources that drive aircompressors, hydraulic pumps, and industrial gas turbine (IGT) powergeneration. Further, the power from turbine engines is used forstationary power supplies such as backup electrical generators forhospitals and the like.

Most turbine engines generally follow the same basic power generationprocedure. Compressed air generated by axial and/or radial compressorsis mixed with fuel and burned, and the expanding hot combustion gasesare directed against stationary turbine vanes in the engine. The vanesturn the high velocity gas flow partially sideways to impinge on theturbine blades mounted on a rotatable turbine disk. The force of theimpinging gas causes the turbine disk to spin at high speed. Jetpropulsion engines use the power created by the rotating turbine disk todraw more air into the engine and the high velocity combustion gas ispassed out of the gas turbine aft end to create forward thrust. Otherengines use this power to turn one or more propellers, fans, electricalgenerators, or other devices.

Low and high pressure compressor (LPC/HPC) components such as compressorblades and impellers are primary components in the cold section for anyturbine engine, and they must be well maintained. The LPC/HPC componentsare subjected to stress loadings during turbine engine operation, andalso are impacted by foreign objects such as sand, dirt, and other suchdebris. The LPC/HPC components can degrade over time due to wear,erosion and foreign object damage. Sometimes LPC/HPC components aredegraded to a point at which they must be repaired or replaced, whichthat can result in significant operating expense and time out ofservice.

There are several traditional methods for repairing damaged turbineengine components, and each method has some limitations in terms ofsuccess. One primary reason for the lack of success is that thematerials used to make LPC/HPC components do not lend themselves toefficient repair techniques. For example, titanium alloys are commonlyused to make fan and compressor blades because the alloys are strong,light weight, and highly corrosion resistant. However, repairing thecompressor blade with conventional welding techniques subjects thecompressor blade to high temperatures at which the welding areas areoxidation-prone. For this reason, welding conventionally is performed ina well-shielded atmosphere such as an inert gas chamber or a chamberthat is under vacuum. Maintaining such a controlled environment isinefficient in terms of both time and expense.

Also, conventional techniques for repairing titanium alloy componentswhich are made of alpha-beta alloys with high beta stabilizers such asTi-6Al-2Sn-4Zr-6Mo possibly cause the components to crack while in awelding zone and/or a heat-affected zone because the alloy componentshave limited weldability. Resistance to cracking can be improved bypreheating the components before welding and then stress relievingimmediately after welding. However, combining preheating and welding isinefficient in terms of both time and expense.

Hence, there is a need for new repair methods for titanium alloycomponents. There is a particular need for new and more efficient repairmethods that improve the reliability and performance of the repairedcomponents.

BRIEF SUMMARY

The present invention provides a method for repairing a titanium alloysurface of a turbine component. The method comprises the step of coldgas-dynamic spraying a powder material comprising at least one titaniumalloy directly on the titanium alloy surface.

In one embodiment, and by way of example only, the method furthercomprises hot isostatic pressing the cold gas-dynamic sprayed turbinecomponent, and a separate heat treating step is performed after the hotisostatic pressing

Other independent features and advantages of the preferred methods willbecome apparent from the following detailed description, taken inconjunction with the accompanying drawings which illustrate, by way ofexample, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an exemplary cold gas-dynamic sprayapparatus in accordance with an exemplary embodiment;

FIG. 2 is a perspective view of an exemplary compressor turbine blade inaccordance with an exemplary embodiment; and

FIG. 3 is a flow diagram of a repair method in accordance with anexemplary embodiment.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The following detailed description of the invention is merely exemplaryin nature and is not intended to limit the invention or the applicationand uses of the invention. Furthermore, there is no intention to bebound by any theory presented in the preceding background of theinvention or the following detailed description of the invention.

The present invention provides an improved method for repairing LPC/HPCcomponents. The method utilizes a cold gas-dynamic spray technique toapply high-strength titanium alloy materials to worn LPC/HPC componentsurfaces. These materials can be used to repair components such ascompressor and fan blades and vanes, including impeller and bliskblades, which have been degraded due to erosion and foreign objectdamage, to name several examples.

Turning now to FIG. 1, an exemplary cold gas-dynamic spray system 100 isillustrated diagrammatically. The system 100 is illustrated as a generalscheme, and additional features and components can be implemented intothe system 100 as necessary. The main components of the cold-gas-dynamicspray system 100 includes a powder feeder for providing repair powdermaterials, a carrier gas supply (typically including a heater), a mixingchamber and a convergent-divergent nozzle. In general, the system 100mixes the repair particles with a suitable pressurized gas in the mixingchamber. The particles are accelerated through the specially designednozzle and directed toward a target surface on the turbine component.When the particles strike the target surface, converted kinetic energycauses plastic deformation of the particles, which in turn causes theparticle to form a bond with the target surface. Thus, the coldgas-dynamic spray system 100 can bond repair powder materials to anLPC/HPC component surface and thereby restore degraded LPC/HPC componentgeometry and dimensions.

The cold gas dynamic spray process is referred to as a “cold gas”process because the particles are mixed and applied at a temperaturethat is far below the melting point of the particles. The kinetic energyof the particles on impact with the target surface, rather than particletemperature, causes the particles to plastically deform and bond withthe target surface. Therefore, bonding to the LPC/HPC component surfacetakes place as a solid state process with insufficient thermal energy totransition the solid powders to molten droplets.

According to the present invention, the cold gas-dynamic spray system100 applies high-strength titanium alloy materials that are difficult toweld or otherwise apply to LPC/HPC component surfaces and other titaniumalloy substrates. For example, titanium alloy welding processes areconventionally performed in a well-shielded atmosphere such as an inertgas chamber or a chamber that is under vacuum. Maintaining such acontrolled environment is inefficient in terms of both time and expense.In contrast, the cold gas-dynamic spray system 100 can be operated atambient temperature and pressure environment.

The cold gas-dynamic spray system 100 is also useful to spray a widevariety of titanium alloys. Near alpha titanium alloys, alpha-plus-betatitanium alloys, and near beta titanium alloys are classes that thesystem 100 can cold spray. Examples of the type of titanium alloys thatcan be cold sprayed using the system 100 include Ti-6Al-4V,Ti-6Al-2Sn-4Zr-6Mo, Ti-6Al-2Sn-4Zr-2Mo, Ti-8Al-1Mo-1V,Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.35Si, as well as specially formulatedand tailored alloys. In an exemplary embodiment of the invention, thecold sprayed titanium alloy is selected to be the same material thatforms the LPC/HPC component to be repaired, although it is clearlywithin the scope of the present invention to select a titanium alloythat is different from the LPC/HPC component material.

As previously mentioned, the cold gas-dynamic spray process can be usedto repair a variety of different turbine engine components. For example,the turbine blades in the high pressure stages of a turbine engine areparticularly susceptible to wear, erosion and other degradation. Turningnow to FIG. 2, a compressor blade 150 that is exemplary of the typesthat are used in turbine engines is illustrated, although compressorblades commonly have different shapes, dimensions and sizes depending ongas turbine engine models and applications. The blade 150 includesseveral components that are particularly susceptible to wear, erosionand foreign object damage, and the process of the present invention canbe tailored to repair different blade components. Among such bladecomponents is an airfoil 152, which is a smooth, curved structure. Theairfoil 152 includes one concave face and one convex face. In operation,air is drawn into the compressor where multiple stages of compressorairfoils act to compress the air in preparation for combustion with sometype of fuel. The airfoil 152 includes a leading edge 162 and a trailingedge 164 that encounter air streaming around the airfoil 152. Thecompressor blade 150 also includes a tip 160. In some applications thetip may include features commonly known as squealers. The compressorblade 150 is mounted on a non-illustrated compressor hub or rotor diskby way of a dovetail 154 that extends downwardly from the airfoil 152and engages with a slot on the compressor hub. A platform 156 extendslongitudinally outwardly from the area where the airfoil 152 is joinedto the dovetail 154. Common features on some compressor and fan bladesare midspan dampers or snubbers 158, which are typically centrallylocated on each side of the airfoil 152. The dampers or snubbers 158extend outwardly to engage with mating features of adjacent compressoror fan blades within the rotor. This engagement makes the dampers orsnubbers 158 common wear features that can be repaired according to themethod of the present invention. Other compressor configurations includeblisks or integrally bladed rotors (IBRs) and impellers or centrifugalcompressors, which have blades that are integral to the rotor hub.

As mentioned previously, the process of the present invention can betailored to fit the blade's specific needs, which depend in part on theblade component where degradation has occurred. For example, the airfoiltip 160 is particularly subject to degradation due to rubbing and othercontact with the static shroud, in addition to foreign particle impacts,and the cold gas dynamic spray process of the present invention is usedto apply materials to the blade tip 160 by filling any material defectswith titanium alloy material. Following the cold spraying process, thetip 160 is machined to restore the tip 160 to the original designdimensions.

As another example, degradation on the leading edge 162 and trailingedge 164 of the airfoil 152 can be repaired using the cold gas-dynamicspray process. The leading edge 162 and trailing edge 164 are bothsubject to degradation, again typically due to tip rubs and foreignparticle impacts. In this application, the cold gas dynamic sprayprocess is used to apply materials that return the edges of thecompressor blade back to the required dimensions. Again, this can bedone by filling the worn surface and other defects with cold gas-dynamicsprayed repair material followed by dimensional restoration andpost-spray processing.

As another example, degradation on the platform 156 can be repairedusing the cold gas-dynamic spray process. In some applications, wear onthe platform 156 occurs at the contact surfaces 166 between adjacentcompressor blades as well as the dovetail contact surface 154. At thoselocations, the friction can cause fretting and other wear. The coldgas-dynamic spray process can be used to fill the worn surface, cracksand other defects on the platform and dovetail to restore the desireddimensions.

Again, the above repair processes are just examples of how a typicaltitanium alloy compressor blade can be repaired by cold gas-dynamicspraying according to the present invention. It is also emphasized againthat compressor blades are just one example of the type of titaniumalloy components that can be repaired using a cold gas-dynamic sprayprocess. For example, many gas turbine engines include a shroudstructure that surrounds a row of compressor blades at the outer radialend of the blades. The shroud, like the blade tips, can be subjecterosion and repaired using the cold gas-dynamic spray process. Otherturbine engine components that can be repaired in such a manner includecompressor stator vanes, vane support structures, rotor nozzles andother LPC/HPC components.

A variety of different systems and implementations can be used toperform the cold gas-dynamic spraying process. For example, U.S. Pat. No5,302,414, entitled “Gas-Dynamic Spraying Method for Applying a Coating”and incorporated herein by reference, describes an apparatus designed toaccelerate materials having a particle size of between 5 to about 50microns, and to mix the particles with a process gas to provide theparticles with a density of mass flow between 0.05 and 17 g/s-cm².Supersonic velocity is imparted to the gas flow, with the jet formed athigh density and low temperature using a predetermined profile. Theresulting gas and powder mixture is introduced into the supersonic jetto impart sufficient acceleration to ensure a particle velocity rangingbetween 300 and 1200 m/s. In this method, the particles are applied anddeposited in the solid state, i.e., at a temperature which isconsiderably lower than the melting point of the powder material. Theresulting coating is formed by the impact and kinetic energy of theparticles which gets converted to high-speed plastic deformation,causing the particles to bond to the surface. The system typically usesgas pressures of between 5 and 20 atm, and at a temperature of up to750° F. As non limiting examples, the gases can comprise air, nitrogen,helium and mixtures thereof. Again, this system is but one example ofthe type of system that can be adapted to cold spray powder materials tothe target surface.

Turning now to FIG. 3, an exemplary method 200 for repairing turbinecomponents is illustrated. This method includes the cold gas-dynamicspray process described above, and also includes additional optionalprocesses to optimize the resulting repairs. As described above, coldgas-dynamic spray involves “solid state” processes to effect bonding andcoating build-up, and does not rely on the application of externalthermal energy for bonding to occur. However, thermal energy may beprovided after bonding has occurred since thermal energy promotesformation of the desired microstructure and phase distribution for therepaired components. Also, additional processing may be necessary tooptimize bonding within the material and many thermo-mechanicalproperties for the material such as the elastic/plastic properties,mechanical properties, thermal conductivity and thermal expansionproperties. In the method 200, additional optional processing includesvacuum sintering, hot isostatic pressing and additional thermaltreatments to consolidate and homogenize the cold gas-dynamic sprayapplied material and to restore metallurgical integrity to the repairedturbine component.

The first step 202 comprises preparing the repair surface on the turbinecomponent. For example, the step of preparing a compressor blade caninvolve pre-machining, degreasing and grit blasting the surface thatneeds to be repaired to remove any oxidation and dirty materials.

The next step 204 comprises performing a cold gas-dynamic spray ofrepair materials on the turbine component. As described above, in coldgas-dynamic spraying, particles at a temperature well below theirmelting temperature are accelerated and directed to a target surface onthe turbine component. When the particles strike the target surface, thekinetic energy of the particles is converted into plastic deformation ofthe particle, causing the particle to form a strong bond with the targetsurface. The spraying step can include the application of repairmaterial to a variety of different components in the turbine engine. Forexample, material can be applied to worn surfaces on compressor blades,impellers, and vanes in general, and to blade tips, knife seals,leading/trailing edges, and platforms. In all these cases, the sprayingstep 204 generally returns the component to its desired dimensions.

With the repair materials deposited directly on the turbine componentsurfaces, the next step 206 comprises performing a vacuum sintering. Invacuum sintering, the repaired turbine component is diffusion heattreated at a desired temperature in a vacuum for a period of time. Thevacuum sintering enables metallurgical bonding to occur across splatinterfaces through elemental diffusion. The vacuum sintering can alsoremove inter-particle micro-porosity, homogenize and consolidate thecold-sprayed buildup via an atom diffusion mechanism. The thermalprocess parameters for the vacuum sintering depend on the titanium alloythat forms the turbine component.

The next step 208 comprises performing a hot isostatic pressing on therepaired turbine component. The hot isostatic pressing (HIP) is a hightemperature, high-pressure process. The HIP process can be performed ata desired temperature that is sufficient to fully consolidate thecold-sprayed buildup and eliminate defects such as porosity.Additionally, the HIP process strengthens bonding between the repairmaterial buildup and the underlying component, homogenizes the appliedmaterials, and rejuvenates microstructures in the base material. Overallmechanical properties such as tensile and stress rupture strengths ofrepaired gas turbine components can thus be dramatically improved withthe HIP process.

As one example of HIP parameters, pressing can be performed for 2 to 4hours at temperatures of between about 1650 and about 1750° F. and atpressures of about 10 to about 15 ksi for most titanium alloys, althoughthe procedure is carried out at up to about 30 ksi for somehigh-temperature titanium alloys. Of course, this is just one example ofthe type of hot isostatic pressing process that can be used to removedefects after the application of repair materials.

In some embodiments, it may be desirable to perform a rapid coolfollowing the HIP process to reduce the high-temperature solution heattreatment aftermath that could otherwise exist. One advantage of therapid cool capability is that the component alloy and the repairmaterial are retained in “solution treated condition,” reducing the needfor another solution treatment operation. In other words, the HIPfollowed by rapid cool can provide a combination of densification,homogenization and solution treat operation. Using this technique canthus eliminate the need for other heat treatment operations.

The next step 210 comprises performing a heat treatment on the repairedcomponent. The heat treatment can provide a full restoration of themechanical properties of turbine components. It should be noted that insome applications it may be desirable to delete the high temperaturesolution treatment if such operation can be accomplished in steps 204and/or 206. However, some examples of heat treatments are describedbelow for applications in which such a treatment is desired ornecessary.

A two-stage heat treatment is applied in a first example, which isuseful for repairing the Ti-6Al-4V alloy, among others. According tothis example, a compressor blade or other component is heated for aboutone hour at a temperature between about 1725 and about 1775° F. Aftercooling the component with water, the component is heated between abouttwo and about eight hours at a temperature between about 900 and about1100° F.

Another two-stage heat treatment is applied in a second example, whichis useful for repairing the Ti-6Al-2Sn-4Zr-6Mo alloy, among others.According to this second example, a compressor blade or other componentis heated for about one hour at a temperature between about 1550 andabout 1650° F. The component is air cooled, and then heated betweenabout four and about eight hours at a temperature between about 1075 andabout 1125° F.

Yet another two-stage heat treatment is applied in a third example,which is useful for repairing the Ti-8Al-1Mo-1V alloy, among others.According to this third example, a component is heated for about onehour at a temperature between about 1800 and about 1850° F. Thecomponent is then cooled with water or oil. The component is then heatedbetween about four and about eight hours at a temperature between about1050 and about 1100° F.

The present invention thus provides an improved method for repairingturbine engine components. The method utilizes a cold gas-dynamic spraytechnique to repair degradation in fan blades, compressor blades,impellers, blisks, and other turbine engine components. These methodscan be used to repair a variety of defects thus can improve the overalldurability, reliability and performance of the turbine enginethemselves.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt to a particularsituation or material to the teachings of the invention withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the invention not be limited to the particular embodiment disclosedas the best mode contemplated for carrying out this invention, but thatthe invention will include all embodiments falling within the scope ofthe appended claims.

1. A method for repairing a titanium alloy surface of a turbinecomponent, the method comprising the step of: cold gas-dynamic sprayinga powder material comprising at least one titanium alloy directly on thetitanium alloy surface.
 2. The method of claim 1, wherein the powdermaterial consists of at least one titanium alloy.
 3. The method of claim1, wherein the powder material comprises at least one titanium alloyselected from the group consisting of near alpha titanium alloys,alpha-plus-beta titanium alloys, and near-beta titanium alloys.
 4. Themethod of claim 1, wherein the powder material comprises an alloy thatis the same alloy that forms the titanium alloy surface.
 5. The methodof claim 1, wherein the cold gas-dynamic spraying is performed in anatmosphere comprising an inert gas.
 6. The method of claim 5, whereinthe inert gas comprises helium.
 7. The method of claim 1, furthercomprising the step of: heating the turbine component at a temperaturesufficiently high to consolidate the sprayed powder material.
 8. Themethod of claim 1, further comprising the step of: performing a vacuumsintering on the turbine component after the cold gas-dynamic sprayingstep.
 9. The method of claim 1, further comprising the step of: hotisostatic pressing the turbine component after the cold gas-dynamicspraying step.
 10. The method of claim 9, wherein the hot isostaticpressing step is performed 2 to 4 hours at temperatures of between about1650 and about 1750° F. and at a pressure of at least 10 ksi.
 11. Themethod of claim 1, further comprising the step of: heat treating theturbine component after the cold gas-dynamic spraying step, the heattreating comprising a first heating step performed for about one hour ata temperature between about 1725 and about 1775° F., followed by asecond heating step performed for between about two and about eighthours at a temperature between about 900 and about 1100° F.
 12. Themethod of claim 11, wherein the titanium alloy surface being repairedcomprises Ti-6Al-4V.
 13. The method of claim 1, further comprising thestep of: heat treating the turbine component after the cold gas-dynamicspraying step, the heat treating comprising a first heating stepperformed for about one hour at a temperature between about 1550 andabout 1650° F., followed by a second heating step performed for betweenabout four and about eight hours at a temperature between about 1075 andabout 1125° F.
 14. The method of claim 13, wherein the titanium alloysurface being repaired comprises Ti-6Al-2Sn-4Zr-6Mo.
 15. The method ofclaim 1, further comprising the step of: heat treating the turbinecomponent after the cold gas-dynamic spraying step, the heat treatingcomprising a first heating step performed for about one hour at atemperature between about 1800 and about 1850° F., followed by a secondheating step performed for between about four and about eight hours at atemperature between about 1050 and about 1100° F.
 16. The method ofclaim 15, wherein the titanium alloy surface being repaired comprisesTi-8Al-1Mo-1V.
 17. The method of claim 1, wherein turbine componentcomprises a compressor blade.
 18. The method of claim 17, wherein thecompressor blade comprises a tip, and wherein the cold gas-dynamicspraying is performed on the tip.
 19. The method of claim 17, whereinthe compressor blade comprises a leading edge, and wherein the coldgas-dynamic spraying is performed on the leading edge.
 20. The method ofclaim 17, wherein the compressor blade comprises a platform, and whereinthe cold gas-dynamic spraying is performed on the platform.
 21. A methodfor repairing a titanium alloy surface of a turbine component, themethod comprising the steps of: cold gas-dynamic spraying a powdermaterial comprising at least one titanium alloy directly on the titaniumalloy surface; hot isostatic pressing the cold gas-dynamic sprayedturbine component; and heat treating the turbine component after the hotisostatic pressing.
 22. The method of claim 21, wherein the powdermaterial consists of at least one titanium alloy.
 23. The method ofclaim 21, wherein the powder material comprises at least one titaniumalloy selected from the group consisting of near alpha titanium alloys,alpha-plus-beta titanium alloys, and near-beta titanium alloys.
 24. Themethod of claim 21, wherein the powder material comprises an alloy thatis the same alloy that forms the titanium alloy surface.
 25. The methodof claim 21, wherein the cold gas-dynamic spraying is performed in anatmosphere comprising an inert gas.
 26. The method of claim 25, whereinthe inert gas comprises helium.
 27. The method of claim 21, furthercomprising the step of: before the hot isostatic pressing step, heatingthe turbine component at a temperature sufficiently high to consolidatethe sprayed powder material.
 28. The method of claim 21, wherein theheat treating step comprises a first heating step performed for aboutone hour at a temperature between about 1725 and about 1775° F.,followed by a second heating step performed for between about two andabout eight hours at a temperature between about 900 and about 1100° F.29. The method of claim 28, wherein the titanium alloy surface beingrepaired comprises Ti-6Al-4V.
 30. The method of claim 21, wherein theheat treating step comprises a first heating step performed for aboutone hour at a temperature between about 1550 and about 1650° F.,followed by a second heating step performed for between about four andabout eight hours at a temperature between about 1075 and about 1125° F.31. The method of claim 30, wherein the titanium alloy surface beingrepaired comprises Ti-6Al-2Sn-4Zr-6Mo.
 32. The method of claim 21,wherein the heat treating step comprises a first heating step performedfor about one hour at a temperature between about 1800 and about 1850°F., followed by a second heating step performed for between about fourand about eight hours at a temperature between about 1050 and about1100° F.
 33. The method of claim 32, wherein the titanium alloy surfacebeing repaired comprises Ti-8Al-1Mo-1V.